Lubricant additive

ABSTRACT

A lubricant formulation. The lubricant formulation includes a polyol-based base lubricant, an ionic liquid, and an organic nanoparticle. The ionic liquid is selected from the group consisting of trihexyltetradecylphosphonium bis(2-ethylhexyl) phosphate, trihexyl(tetradecyl)phosphonium bis-2,4,4-(trimethylpentyl)phosphinate, and trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)imide, or a combination thereof. The organic nanoparticle has a median particle size less than about 200 nm. The organic nanoparticle forms about 0.01 to about 5% by weight of the lubricant formulation. The ionic liquid forms about 0.5 to about 10% by weight of the lubricant formulation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of a co-pending U.S. patentapplication Ser. No. 14/821,099 filed Aug. 7, 2015, which claimspriority to U.S. Provisional Patent Application No. 62/037,438 filedAug. 14, 2014, the entire contents of which are incorporated byreference herein.

GOVERNMENT LICENSE RIGHTS

This invention was made with government support under Contract No.W911QX-13-C-0174. The government has certain rights in the invention.

TECHNICAL FIELD

The present invention relates to lubricant additives and formulations,and more particularly to lubricant additives and formulations for use inconnection with rotorcraft transmission and gearbox systems, and otherrotary platforms.

BACKGROUND

Metal parts in close tolerances and contacts are a design feature ofmany electromechanical and mechanical devices. Lubricants maintainviscosity and protect components more effectively under the high shearstresses that these systems place on metal parts. The benefits of awell-lubricated system include an increase in the effective service lifeof the constituent parts of the system and the system as a whole, aswell as enhanced fuel efficiency, which can lead to significant costsavings. In a typical engine set-up, 10-15% of the energy is lost due tofriction.

Certain systems, such as rotorcraft transmission systems and rotaryplatforms more generally, are frequently operated in extreme conditionswhich require the use of a high quality lubricant capable of carrying ahigh load. This is especially true in the context of transmission andgearbox systems for military and civilian rotorcraft, as the maingearbox is one of the most vulnerable portions of the rotorcraft. Thisis true even if redundant systems are employed to provide emergencylubrication systems, which add additional weight, complexity, and a riskof dormant failure. When such redundant systems fail, these failurescause both a dangerous situation as well as widespread inconvenience forboth the operators of the rotorcraft as well as those served by therotorcraft, such as offshore workers. In some countries, including theU.S., the use of a lubricant capable of supporting an aircraft in safeflight for at least 30 minutes after the crew has detected lubricationsystem failure or loss of lubrication is required for use in certaincontexts.

Accordingly, an improved lubrication system, deliverable throughconventional service channels is therefore desirable. Ionic liquids havebeen known to enhance the lubricity of a system/material, for example asdisclosed in U.S. Pat. Nos. 8,318,644 and 7,754,664, and the article“Ionic Liquids in Tribology” (Minami, Ichirio, Molecules 14, no. 6(2009): 2286-2305), each of which is incorporated by reference herein inits entirety. Due to the inherent polarity of ionic liquids, they adsorbstrongly on the metallic tribocontact surfaces leading to a robusttribofilm when compared to conventional lubricants. However ionicliquids have an intrinsically high cost. Also, the use of some ionicliquids having halogens can also result in undesirable corrosion ofmetal surfaces having specific compositions.

Metal nanoparticles have also emerged as an approach to advanceddevelopment for enhanced lubrication and heat transfer capability. Forexample, incorporating metal nanoparticles into the tribofilm canenhance rolling friction between the contact surfaces, thereby reducingwear.

SUMMARY

In one aspect, an additive composition is disclosed. The additivecomposition includes an ionic liquid and an organic nanoparticle.

In another aspect, a lubricant formulation is disclosed. The lubricantformulation includes a base lubricant, an ionic liquid, and an organicnanoparticle.

In yet another aspect, a lubricant formulation is disclosed. Thelubricant formulation includes a polyol-based base lubricant, an ionicliquid, and an organic nanoparticle. The ionic liquid is selected fromthe group consisting of trihexyltetradecylphosphonium bis(2-ethylhexyl)phosphate, trihexyl(tetradecyl)phosphoniumbis-2,4,4-(trimethylpentyl)phosphinate, andtrihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)imide, or acombination thereof. The organic nanoparticle has a median particle sizeless than about 200 nm. The organic nanoparticle forms about 0.01 toabout 5% by weight of the lubricant formulation. The ionic liquid formsabout 1 to about 10% by weight of the lubricant formulation.

Other aspects of the disclosed additive composition and lubricantformulation will become apparent from the following description, theaccompanying drawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments set forth in the drawings are illustrative and exemplaryin nature and not intended to limit the subject matter defined by theclaims. The following detailed description of the illustrativeembodiments can be understood when read in conjunction with thefollowing drawings.

FIG. 1 is a chart showing comparative friction coefficient profiles forembodiments of a lubricant formulation.

DETAILED DESCRIPTION

For simplicity and illustrative purposes, the principles of the presentinvention are described by referring to various exemplary embodimentsthereof. Although the preferred embodiments of the invention areparticularly disclosed herein, one of ordinary skill in the art willreadily recognize that the same principles are equally applicable to andcan be implemented in other systems, and that any such variation wouldbe within such modifications that do not part from the scope of thepresent invention. Before explaining the disclosed embodiments of thepresent invention in detail, it is to be understood that the inventionis not limited in its application to the details of any particulararrangement shown, since the invention is capable of other embodiments.The terminology used herein is for the purpose of description and not oflimitation.

An additive composition for a base lubricant is disclosed including oneor more ionic liquids and one or more organic nanoparticles. Lubricantformulations incorporating the disclosed additive provide enhancedperformance in terms of wear protection of system parts, reducedcoefficient of friction, lower electrical resistance, and longer oil-outrun time as compared to the performance of the base lubricant alone,under identical operating conditions.

The term “base lubricant,” as used herein, may refer to an unformulatedlubricant or a fully-formulated lubricant with additives added thereto,including but not limited to commercially-available formulated and/orunformulated lubricants.

The base lubricant may be any of a variety of base lubricants known inthe art, or combinations thereof, including but not limited to baselubricants conventionally used in any of a variety of applications,including lubrication of engines and/or rotorcraft transmission andgearbox systems, such as natural or synthetic oils. In one embodiment,the base lubricant may be a polyol ester or a polyol-based lubricantincluding hindered polyol esters and any of a variety of additives, andit may be a commercially-available base lubricant approved for use underU.S. military specification DOD-L-85734. For example, the base lubricantmay be AEROSHELL® Turbine Oil 555, which is commonly used in currentrotorcraft systems. Other non-limiting examples of base lubricantsinclude but are not limited to transmission oils such as Herco A (polyolester, unformulated) and MOBIL SHC® 626 (formulated) and internalcombustion engine oils such as mineral oil (unformulated) and MOBIL 1™5W-30 (formulated).

The ionic liquid of the additive composition may be any of a variety ofionic liquids, or combinations thereof. The addition of an ionic liquidto the base lubricant appears to facilitate rapid formation of aprotective tribocoating on metal surfaces of the system incorporatingthe lubricant. In one embodiment, the ionic liquid is a non-corrosiveionic liquid, such as a halogen-free ionic liquid, to reduce wear onsystem parts. The halogen-free nature of the ionic liquid reducessensitivity for hydrolysis, which in turn reduces the incidence ofcorrosion. Ionic liquids are known in the art, and selection of asuitable ionic liquid may be based on factors such as lubricity and theability to protect against corrosion. Under given test conditions gearsteel (for example, AISI 9310 alloy steel) with the ionic liquid mayhave a coefficient of friction less than that of the base lubricant. Inone non-limiting example (ball-on-disc test, Hertzian stress 800 MPa),the ionic liquid is trihexyltetradecylphosphonium bis(2-ethylhexyl)phosphate, which yields a coefficient of friction of about 0.044 withAISI 9310 alloy steel, as compared to AEROSHELL® 555, which yields acoefficient of friction of about 0.057 with AISI 9310 alloy steel.

Representative ionic liquids that may be used include phosphonium-basedionic liquids such as trihexyltetradecylphosphonium bis(2-ethylhexyl)phosphate, trihexyl(tetradecyl)phosphoniumbis-2,4,4-(trimethylpentyl)phosphinate andtrihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)imide. Oneof ordinary skill will appreciate that other ionic liquids known in theart, including non-corrosive ionic liquids, may be incorporated into theadditive composition alone or in combination without departing from thescope of this disclosure. The additive composition may be incorporatedinto the lubricant formulation such that the ionic liquid is provided inthe lubricant formulation in an amount of about 0.01-15% by weight, orvarious embodiments, about 0.01-1.0%, about 0.01-2.0%, about 0.01-3.0%,about 0.01-4.0%, about 0.01-5.0%, about 0.01-6.0%, about 0.01-7.0%,about 0.01-8.0%, about 0.01-9.0%, about 0.01-10.0%, about 0.5%-10.0%,about 1.0%-5.0%, about 1.0%-6.0%, about 1.0%-7.0%, about 1.0%-8.0%,about 1.0%-9.0%, about 1.0%-10.0%, about 1.0%-15.0%, about 2.0%-6.0%,about 3.0%-6.0%, about 2.0-10.0% by weight, about 4.0-6.0%, about 1%,about 2%, about 3%, about 4% about 5%, about 6%, about 7%, about 8%,about 9%, or about 10% by weight.

The organic nanoparticles of the additive composition may be any of avariety of carbon-based or carbon-containing nanoparticles, orcombinations of multiple varieties of nanoparticles, including but notlimited to nanographene (including nanographene platelets), grapheneoxide, carbon, carbon nanotubes (single, double, or multi-walled),carbon nanofibers, fullerenes, nanodots, nanopowders, nano-diamond andthe like, in any of a variety of morphological configurations. Carbonnanoparticles are less expensive than metal nanoparticles of metals suchas copper, silver, and gold, and carbon nanoparticles may be less toxicand safer to handle than metal-based nanoparticles. The organicnanoparticles may range in size from about 0.1 to 999 nm in medianparticle size, and in one embodiment no greater than about 200 nm inmedian particle size. The nanoparticles may include mesopores and/ormicropores, which may improve buoyancy of the nanoparticles within theresultant lubricant formulation and prevent settling. The additivecomposition may be incorporated into the lubrication formulation suchthat the organic nanoparticles are provided in the lubricant formulationin the amount of about 0.01-10% by weight, or in various embodiments,about 0.01%, about 0.02%, about 0.03%, about 0.04%, about 0.05%, about0.06%, about 0.07%, about 0.08%, about 0.09%, about 0.10%, about0.01-0.03%, about 0.01-0.04%, about 0.01-0.05%, about 0.01-0.06%, about0.01-0.07%, about 0.01-0.08%, about 0.01-0.09%, about 0.01-0.10%, about0.01-1.0%, about 0.01-2.0%, about 0.01-5.0%, about 0.05-0.5%, or about0.1-1.0% by weight.

The ranges disclosed herein with respect to the ionic liquid content andthe organic nanoparticle content of the additive compositions may beinterchangeably combined in any combination, with any ionic liquid ororganic nanoparticle disclosed herein. For example, the additivecomposition of the lubricant formulation may, in one embodiment, includeabout 1.0-8.0% by weight ionic liquid (trihexyltetradecylphosphoniumbis(2-ethylhexyl) phosphate) and about 0.01-0.10% by weight organicnanoparticle (graphene platelets), and in another embodiment, about 5%by weight ionic liquid (trihexyl(tetradecyl)phosphoniumbis-2,4,4-(trimethylpentyl)phosphinate) and about 0.01-5% organicnanoparticle (carbon nanotubes). Each permutation of the embodiments ofthese ranges may be further used in combination with any of the baselubricants described herein.

Without wishing to be bound by the theory, when the lubricationformulation incorporating the additive composition is utilized in anengine system, the organic nanoparticles aggregate in wear grooves,patterns, and/or facets in the surfaces of the parts being lubricatedthat may form during the operation of the system or otherwise, therebyhaving a mending effect on the pertinent surfaces as the nanoparticlesaccumulate. Accordingly, the organic nanoparticles may providelubrication and hence additional protection to the system even withoutthe presence of the liquid lubricant components (i.e. the base lubricantand/or the ionic liquid additive component), for example if the liquidlubricant components are lost or removed for any reason, followed by theloss of the ionic liquid-induced tribocoating. This improves the abilityof the lubricant formulation to provide protection to system parts evenin the event of a lubrication failure or the loss of lubricant duringoperation.

The disclosed additive composition therefore provides a number ofbenefits over state of the art lubricants because it provides at leastthe dual benefits of rapidly establishing the triboprotective coating onsystem surfaces via the ionic liquid, and also synergistically fillingin irregularities on the metal surfaces to be lubricated via aggregationof the organic nanoparticles. Together, these dual benefits greatlyenhance the ability of a system, such as a rotorcraft, to continue tooperate safely post-lubrication system failure or lubricant loss for asignificantly longer period of time than a base lubricant lacking theionic liquid and organic nanoparticle components of the additivecomposition. Further, because the additive composition provides thesebenefits as an additive to a relatively inexpensive base lubricant,there is significant cost savings as compared to formulating lubricantscomposed primarily of an expensive ionic liquid base.

The additive composition enhances the function of formulation used forboth internal combustion engines and also transmission lubrications, andis therefore suitable for a wide variety of applications beyondrotorcraft transmission and gearbox systems, such as use in bearingapplications and/or other tribomechanical systems that requirelubrication.

In one non-limiting example, the additive composition included carbonnanoparticles and the ionic liquid trihexyltetradecylphosphoniumbis(2-ethylhexyl) phosphate, which were added to the base lubricant ofAEROSHELL® 555 in the amounts of 5.0% by weight ionic liquid, 0.1% byweight carbon nanoparticle, and 94.9% by weight AEROSHELL® 555. Aprotocol for an oil-out simulation was created on the Cameron-Plinttribometer to test the effectiveness of this lubricant formulation. Forthe first 5 minutes, the test was run at 20N load as a run-in period ina fully flooded (2 ml of lubricant formulation) condition. After 5minutes, the load was increased to 250N (Hertzian stress 700 MPa). Aftera 30 minute run with the 250N load, an oil-out event was simulated bycompletely removing the lubricant formulation. The test was continuedunder the “oil-out” condition. For each run, the test was terminatedwhen the friction coefficient increased to 0.3, or the test duration(typically 300 minutes) ended. The tribological performance of thelubricant formulation was compared with AEROSHELL® 555 (base line) undersuch simulated oil-out conditions. The increase in run time afteroil-out test in the lubricant formulation was greater than 2108% of thebase line result.

In another non-limiting example, and with reference to FIG. 1, theadditive composition included nano-graphene platelets and the ionicliquid trihexyltetradecylphosphonium bis(2-ethylhexyl) phosphate, whichwere added to the base lubricant of AEROSHELL® 555 in the amounts of 1%,3%, and 5.0% by weight ionic liquid, 0.02% by weight graphene, and98.98%, 96.98% and 94.98% by weight AEROSHELL® 555. A protocol for theoil-out simulation was created on the Cameron-Plint tribometer to testthe effectiveness of these lubricant formulations. In each case, for thefirst 5 minutes, the test was run at 20N load as a run-in period in afully flooded (1 ml of lubricant formulation) condition. After 5 minutesthe load was increased to 250N (Hertzian stress 700 MPa). To create theoil-out event, the lubricant was completely removed after a 60 minuterun with the 250N load, and the test was continued under the “oil-out”condition. In FIG. 1, the “oil-out” time is represented by the hash markat 60 minutes on the x-axis. The test was terminated when the frictioncoefficient increased to 0.3, or the test duration (typically 300minutes) ended. As shown in FIG. 1, the friction coefficient of certainlubricant formulations rises sharply (>0.3) after a certain amount oftime in an oil-out condition. The tribological performances of the threedifferent lubricant formulations were compared with AEROSHELL® 555 (baseline) under such simulated oil-out conditions. The results are detailedin Table 1, below:

TABLE 1 Average Time from Increase Average Wear Oil-Out Until in RunLubricant Friction Reduction Friction Time After Formulation Coefficient% Coefficient >0.3 Oil-Out, % AEROSHELL ® 0.12 — 12.65 minutes — 555(baseline) AEROSHELL ® 0.13   3% 25.55 minutes  102% 555, with 1% ionicliquid and 0.02% organic nanoparticle AEROSHELL ® 0.12 −32% 143.11minutes  1031% 555, with 3% ionic liquid and 0.02% organic nanoparticleAEROSHELL ® 0.11 −35% >304.67 minutes >2308% 555, with 5% ionic liquidand 0.02% organic nanoparticle

As shown in Table 1, the 1% ionic liquid formulation provided similarresults to the baseline in terms of lubricity and wear, but more thandoubled the effective run time of the engine after lubricant removal ascompared to the baseline test. Each of the 3% and 5% ionic liquidformulations provided both significant wear reduction and alsosignificant improvements in run time—at least about 10 to 25 times thebaseline without the additive composition.

The effectiveness of carbon nanoparticles to reduce wear was alsotested. Under fully-flooded conditions, about 35% reduction of wear wasobserved for a blend of AEROSHELL® 555+0.1% carbon nano-particle ascompared to baseline of AEROSHELL® 555, alone, under the sameconditions.

While the invention has been described with reference to certainexemplary embodiments thereof, those skilled in the art may make variousmodifications to the described embodiments of the invention withoutdeparting from the scope of the invention. The terms and descriptionsused herein are set forth by way of illustration only and not meant aslimitations. In particular, although the present invention has beendescribed by way of examples, a variety of compositions and processeswould practice the inventive concepts described herein. Although theinvention has been described and disclosed in various terms and certainembodiments, the scope of the invention is not intended to be, norshould it be deemed to be, limited thereby and such other modificationsor embodiments as may be suggested by the teachings herein areparticularly reserved, especially as they fall within the breadth andscope of the claims which are to be appended. Those skilled in the artwill recognize that these and other variations are possible within thescope of the invention as defined in the claims and their equivalents.

What is claimed is:
 1. A method of lubricating at least two metallictribo-contact surfaces, the method comprising the steps of: applying abase lubricant between the metallic tribo-contact surfaces; and afterapplying the base lubricant between the metallic tribo-contact surfaces,adding an additive to the base lubricant, the additive comprisingorganic nanoparticles dispersed in an ionic liquid.
 2. The method ofclaim 1, wherein the ionic liquid is halogen-free.
 3. The method ofclaim 2, wherein the ionic liquid is a phosphonium-based ionic liquid.4. The method of claim 3, wherein the ionic liquid is selected from thegroup consisting of trihexyltetradecylphosphonium bis(2-ethylhexyl)phosphate, trihexyl(tetradecyl)phosphoniumbis-2,4,4-(trimethylpentyl)phosphinate, andtrihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)imide, andcombinations thereof.
 5. The method of claim 1, wherein the organicnanoparticle is graphene or other submicron particle of carbon.
 6. Themethod of claim 1, wherein the organic nanoparticle has a medianparticle size less than about 200 nm.
 7. The method of claim 3, whereinthe base lubricant is a polyol-based lubricant.
 8. A method oflubricating an aircraft to extend the duration of safe flight during aloss of lubrication event, the method comprising the steps of: addingorganic nanoparticles dispersed in an ionic liquid to an aircraftlubrication system.
 9. The method of claim 8, wherein the organicnanoparticles and the ionic liquid are added to the aircraft lubricationsystem as a base lubricant additive.
 10. The method of claim 8, whereinthe ionic liquid is halogen-free.
 11. The method of claim 8, wherein theionic liquid is a phosphonium-based ionic liquid.
 12. The method ofclaim 8, wherein the organic nanoparticle is graphene or other submicronparticle of carbon.
 13. A lubrication system comprising: a firstmetallic surface; a second metallic surface; and a protectivetribocoating formed between the first and second metallic surfaces, theprotective tribocoating comprising a polyol-based lubricant, an ionicliquid, and organic nanoparticles.
 14. The lubrication system of claim13, wherein the ionic liquid is halogen-free.
 15. The lubrication systemof claim 13, wherein ionic liquid is a phosphonium-based ionic liquid.16. The lubrication system of claim 13, wherein the organic nanoparticleis graphene or other submicron particle of carbon.